As a corrugated metal pipe culvert or storm sewer ages, it is common for the invert to bear the brunt of the corrosion and abrasion damage. Paving the invert with concrete can be an inexpensive way to extend the service life of the structure if done right and at the right time. A non-reinforced paved invert with no mechanical attachments could provide 15 - 25 or more years of additional service in non-freeze thaw zones and in existing pipes that are at a abrasion level 1 (as defined by FHWA). Be careful though, in freeze thaw zones or with those pipe structures that would be considered a level 2, 3 or 4 for abrasion, a non-reinforced and unattached paved invert could provide very little additional service life.
3 THINGS TO CONSIDER:
- Can I use a plain concrete pavement? - As a general rule of thumb the amount of wall section loss shouldn't be more than 1/4. That means roughly 3/4 of the original wall thickness should still be present in order for a plain, non-reinforced and non-anchored concrete pavement to work. The original structural design can provide the baseline factors of safety to help with this analysis and each particular structure location can be evaluated using current industry methods to get precise results, but you’ll need a representative number of core samples from the invert of the host pipe in order to truly know the average amount of metal loss in the invert. Mechanical attachments may not be necessary if the host pipe is still operating within its original design safety factors.
- What if the pipe is already perforated or badly corroded? - If the pipe has already perforated uniformly, the pavement must incorporate the use of reinforcement and thrust transfer mechanisms. Mechanical transfer of the actual load or thrust carried by the original pipe wall is important in order for the pipe to continue to function properly. Flexible CMP structures rely on a phenomenon known as ‘soil-structure interaction’. If significant metal loss has occurred, or migration of any of the backfill material has occurred, or if the pipe has begun to move, then a full thrust transfer is critical because soil-structure interaction is no longer assumed to be present.
- How do I design a structural paved invert? - Caltrans has established a straightforward and effective way to accommodate thrust transfer. It is detailed in DIB 83, which is a design information bulletin. It is a solid technical resource from a very reputable DOT. Also, it is easy to find online.
A paved invert (both non-structural and structural) can be a very cost effective way to extend the service life of a CMP culvert or storm sewer. I hope this was helpful !